The present invention generally relates to radio communication apparatus such as cellular phones, and semiconductor devices incorporated in the radio communication apparatus, and particularly to a radio communication apparatus in which a high-frequency power amplifier module of a single-stage arrangement using a single high-power amplifier that has a plurality of transistors connected in parallel or of a multi-stage arrangement using a plurality of such high-power amplifiers of cascade connection is provided at the transmitter-side output stage, and in which an antenna is provided to be connected to this high-frequency power amplifier module, so that prevention of thermal runaway, improvement of efficiency and prevention of oscillation can be achieved.
The mobile communication apparatus (radio communication apparatus) such as car phone and cell phone has a high-frequency power amplifier module (high-frequency power amplifier circuits) built in its transmitter-side output stage. The output (transmission power) of this high-frequency power amplifier module is automatically controlled by a bias controlling circuit (APC: Automatic Power Control).
In general, a system for portable telephone is constructed in which the output from a cellular phone is changed to conform to the surrounding environment according to a power level indicating signal transmitted from a base station, thereby preventing the cross talk to or from other cellular phones. In the cellular phone, its bias control circuit supplies a certain signal (control signal) according to the received power level indicating signal to the high-frequency power amplifier module, thereby making the output from the radio communication apparatus be adjusted.
The high-frequency power amplifier module is of a single stage arrangement using a single semiconductor amplifier element (transistor) or of a multi-stage arrangement using a plurality of such transistors connected in cascade. The transistors used are bipolar transistors, MOS FET (Metal Oxide Semiconductor Field-Effect-Transistor), GaAs-MES (Metal-Semiconductor) FET, HEMT (High Electron Mobility Transistor), and HBT (Heterojunction Bipolar transistor).
U.S. Pat. No. 5,629,648 (issued May 13, 1997) discloses a high power amplifier circuit using a heterojunction bipolar transistor. This document describes that a high power bipolar transistor circuit can suppress the thermal runaway by providing a resistor for DC bias and a capacitor for AC signal at the base of each of a plurality of transistors connected in parallel, and by the DC voltage drop across the resistor.
In JP-A-7-7014 laid open Jan. 10, 1995 (corresponding to U.S. Pat. No. 5,321,279 issued Jun. 14, 1994) as another example of the prior art, it is described that a power HBT can suppress the thermal runaway (current hogging: hot-spot) by connecting a ballast high impedance to the base fingers of a Si bipolar transistor of multi-finger structure having emitter fingers (emitter terminals), base fingers (base terminals) and collector fingers (collector terminals) in order that the apparatus can be operated with high reliability. That is, the hot-spot formation due to current concentration can be suppressed by providing a ballast impedance (resistor) at each base finger.
In this document, a capacitor (bypass capacitor) is also connected in parallel to the ballast impedance (resistor) in order to assure the minimum gain loss of transistor. It is also described that the power HBT mentioned above can make a high power amplifier and can be used for cellular phone.
In addition, JP-A-7-94975 (laid open Apr. 7, 1995) discloses a high-frequency HIC module (high-frequency power amplifier module) of three-stage configuration having cascade-connected MOS FETs. This high-frequency HIC module is constructed to have a first bias circuit that biases the gate of a certain one of a plurality of MOS FETs according to the output control voltage, a second bias circuit that biases the gates of the other MOS FETs than the certain one on the basis of a fixed power source, and switching means that switches the paths of the fixed power source and the second biasing circuit on the basis of the output control voltage. This module thus increases the controllability of the output, and improves the efficiency. In addition, each biasing circuit is formed of three resistors and one capacitor.
The HBT is being used as a semiconductor amplifying element of a high-frequency power amplifier module incorporated in a radio communication apparatus such as a cellular phone because it has excellent characteristics of high speed and low power consumption.
The inventors examined the means for compressing both thermal runaway and oscillation phenomenon of HBT element and at the same time for improving the efficiency while they were developing a HBT power amplifier module (high-frequency power amplifier module) for cellular phone. From the examination, it was confirmed that the radio communication apparatus employing the well-known circuits caused the following drawbacks.
FIG. 18 is a schematic circuit diagram of part of a radio communication apparatus having incorporated therein a single-stage amplifier (high-frequency power amplifier module) using a single HBT of multi-finger structure. This module is the same as the circuit arrangement (conventional example 1) described in U.S. Pat. No. 5,629,648. Here, for the sake of explanation, a portion formed of an emitter terminal (emitter finger), a base terminal (base finger) and a collector terminal (collector finger) is called a transistor (unit cell), and a group of such transistors connected in parallel as a multi-finger structure transistor or multi-finger transistor. Thus, the above-mentioned HBT is one multi-finger transistor.
A high-frequency power amplifier module 1 has, as external terminals, an input terminal (RF in), an output terminal (RF out), a first voltage terminal (Vcc) serving as the collector terminal, too, a second voltage terminal (ground: GND) serving as the emitter terminal, too, and a biasing terminal (control terminal: Vapc) serving as the base terminal, too. The output terminal (RF out) is connected to an antenna 2 through a filter or the like not shown.
The HBT is a multi-finger transistor that has N transistors connected in parallel. The transistors, Q1Axcx9cQ1N each has emitter terminal 5, base terminal 6 and collector terminal 7.
In this circuit arrangement, the input terminal and the control terminal are separated. Junction capacitors C2Axcx9cC2N are connected between the input terminal and the base terminals 6 of the transistors Q1Axcx9cQ1N, and ballast resistors R2Axcx9cR2N between the control terminal and the base terminals 6 of the transistors Q1Axcx9cQ1N.
In addition, the first voltage terminal (Vcc) and the collector terminals 7 are connected to a single inductor Lc, and the emitter terminals 5 to ground (GND). The high-frequency power amplifier module 1 also has a matching circuit 9 provided at the output side in order to match with the impedance of the antenna 2.
In this radio communication apparatus, since an AC signal is supplied through only capacitors, when the efficiency of the apparatus is raised to about 60% (corresponding to about 70% of the efficiency of the high-frequency power amplifier module), the element stability becomes poor, and oscillation phenomenon is apt to occur because of no loss in the signal paths, with the result that stable communication might be lost. In other words, the impedance viewing the external circuit at high frequencies is small, and the stabilization coefficient K becomes noticeably small, thus the transistors being unstable.
FIG. 6 shows a graph of the correlation between the stability coefficient K and collector current. In this graph, the curve indicates the stability coefficient of conventional example 1. From the curve, it will be seen that the stability coefficient K is about 0.15 relative to a collector current of 0.01 A and further decreases with the increase of collector current.
FIG. 7 shows a graph of the relation between the input power (dBm) and power added efficiency (%). In this graph, the curve indicates the power added efficiency of conventional example 1. From the curve, it will be seen that the power added efficiency reaches the maximum value of about 68.5% at an input power of about 25 dBm.
FIGS. 19 and 20 are schematic circuit diagrams of part of the radio communication apparatus that has incorporated therein a single-stage amplifier (high-frequency power amplifier module) using one HBT of multi-finger structure. Those circuit arrangements are the same as the circuit arrangement described in U.S. Pat. No. 5,321,279 given above. In the circuit arrangement of FIG. 19, ballast resistors R1Axcx9cR1N are simply connected between the control terminal (Vapc) and the base terminals 6 of transistors Q1Axcx9cQ1N. However, there is a fear that, if the ballast resistors have enough values for thermal runaway prevention, the high-frequency signal attenuates with the result that the amplification characteristic is remarkably deteriorated.
In the circuit arrangement (conventional example 2) of FIG. 20, bypass capacitors C1Axcx9cC1N are respectively connected in parallel with resistors R1Axcx9cR1N in order to prevent the high-frequency signal from attenuating. However, in order to reduce the deterioration of the performance, it is necessary that the values of the bypass capacitors be increased (for example, be about 50 pF), thus leading to a large area of the semiconductor chip in which the HBT is built. In addition, since the number of semiconductor chips that can be produced from a single semiconductor substrate (wafer) decreases, the production cost of the semiconductor chip (semiconductor element, or semiconductor device) increases. Moreover, it was found that, since the impedance viewing the external circuit decreased at high frequencies, the stability coefficient K decreased, making it easy to oscillate the circuit. The stability coefficient K, as shown in FIG. 6, is better than in the conventional example 1, but far from the stability coefficient of 1 at which oscillation is difficult to occur. The stability coefficient K relative to a collector current of 0.01xcx9c1 A ranges from about 0.5 to 0.75.
The capacitors C1Axcx9cC1N connected in parallel with the resistors R1Axcx9cR1N are provided to increase the high-frequency gain. However, use of large capacitance values for the gain increase effect will make it easy to oscillate. On the contrary, if the capacitance values are small, the effect to increase the high-frequency gain decreases, making the optimum design difficult.
Accordingly, it is an object of the invention to provide a radio communication apparatus in which oscillation phenomenon is difficult to occur even in a high efficiency region of the apparatus, and a semiconductor device (semiconductor element) incorporated in the radio communication apparatus.
It is another object of the invention to provide a radio communication apparatus that can suppress thermal runaway, operate at high efficiency and make it difficult to oscillate, and a semiconductor device (semiconductor element) incorporated in the radio communication apparatus.
The above objects, other objects and noble features of the invention will be apparent from the description of this specification and the accompanying drawings.
The major features of the embodiments of the invention disclosed in this application include the following features.
(1) A radio communication apparatus having a high-frequency power amplifier module in which a single-stage amplifier using one multi-finger type heterojunction bipolar transistor (HBT) or a multi-stage amplifier using a plurality of HBTs sequentially connected in cascade is incorporated, at the transmitter-side output stage, and an antenna connected to the high-frequency power amplifier module, wherein first capacitors and first resistors are inserted in series between the input terminal of the high-frequency power amplifier module and the control terminals of the HBT, and second resistors are inserted between the control terminal of the high-frequency power amplifier module and the control terminals of the HBT, and connected to the nodes of the first resistors and the first capacitors.
A semiconductor device (semiconductor element) incorporated in the radio communication apparatus includes an input terminal, an output terminal, a bias terminal, a power amplifier that has a plurality of transistors each having first and second terminals and a control terminal for controlling the current flowing between the first and second terminals, first capacitors C2Axcx9cC2N inserted between the input terminal and the control terminals of the transistors, and connected to the input terminal, first resistors R1Axcx9cR1N connected in series with the first capacitors C2Axcx9cC2N and connected to the control terminals, and second resistors R2Axcx9cR2N inserted between the bias terminal and the control terminals of the transistors, and connected to the nodes of the first resistors R1Axcx9cR1N and the first capacitors C2Axcx9cC2N, the output terminal being connected to the first terminals of the transistors.
(2) In the arrangement of the above means (1), second capacitors are respectively connected in parallel with the first resistors. In the semiconductor device incorporated in the radio communication apparatus, the arrangement of the above means (1) also has second capacitors C1Axcx9cC1N connected in parallel with the first resistors R1Axcx9cR1N.
(3) In the above means (1) or (2), an inductor is connected to the base terminals and/or emitter terminals of the transistors.
(4) A radio communication apparatus in which a high-frequency power amplifier module that has incorporated therein a single-stage amplifier using one multi-finger type heterojunction bipolar transistor (HBT) or a multi-stage amplifier using a plurality of HBTs sequentially connected in cascade is provided at the transmitter-side output stage, and an antenna is provided to be connected to the high-frequency power amplifier, wherein the thermal resistance difference between a plurality of terminals (fingers) of the HBT that constitutes one stage of the above amplifier is designed to be much smaller than the average thermal resistance of the fingers, first resistors are connected between the input terminal of the high-frequency power amplifier module and the control terminals of the HBT, first capacitors are connected between an input terminal of the high-frequency power amplifier module and a first node to which the first resistors are connected, and second resistors are connected between the control terminal of the high-frequency power amplifier module and the nodes of the first resistors and capacitors.
According to the above means (1), (a) since the second resistors cause a base voltage drop with the increase of base current due to the temperature rise of the HBT chip, the currents in the transistors can be suppressed from increasing, thus thermal runaway being suppressed. Therefore, the semiconductor device (semiconductor element) can have the same effect as above.
(b) The first resistors cause high-frequency signal loss to prevent the gains from excessively increasing, thus stabilizing the transistors and preventing them from oscillation. Therefore, the semiconductor device can have the same effect as above.
(c) The radio communication apparatus can achieve high efficiency by connecting the first and second resistors and the first capacitors. For example, if the high-frequency power amplifier module is constructed by HBT with about 100 transistors and with emitter size of about 2 xcexcmxc3x9720 xcexcm, it can produce output of about 4 W. At this time, if the current amplification factor is about 80, efficiency of about 70% can be attained under the selection of first capacitors of about 0.15 pF, first resistors of about 100 ohms and second resistor of about 1 kilo ohms (see FIGS. 7 and 8). At the time of this high efficiency operation, the DC current flowing in the first resistors ranges from about 0.2 to 0.5 mA, and the voltage drop across the first resistors is as sufficiently small as about 20 to 50 mV, so that the loss in the first resistors is small. From FIG. 9, it will be seen that the efficiency change is relatively small up to 2.5 times the first resistors of 100 ohms, or 250xcexa9 (parallel resistance of 100 resistors is 2.5xcexa9). The voltage drop across the first resistors at this time is as sufficiently small as 50 to 25 mV. This means that the radio communication apparatus can attain a high efficiency of about 60%.
(d) Since the amplifier can be prevented from simply causing thermal runaway and oscillation due to temperature change and source voltage fluctuation, radio communication can be stably performed without interference.
(e) Although the high-frequency gain can be increased by connecting capacitors in parallel with the first resistors, increase of the capacitance value will cause oscillation. In this invention, since capacitors are not connected in parallel with the first resistors, the oscillation due to the base ballast resistors can be suppressed. Therefore, the semiconductor device has the same effect as above.
According to the means (2), (a) since the first and second resistors connected in series are connected to the base terminals, and second capacitors are connected in parallel with the first resistors, the resistance value of the first resistors can be reduced. As a result, the bypass capacitors (second capacitors) connected in parallel with the first resistors can also be reduced. The capacitance value of the bypass capacitors was about 50 pF in the prior art, but in this invention it can be reduced to about {fraction (1/10)} that value, or 5 pF. Thus, the chip area can be reduced the more. Therefore, the production cost of the semiconductor element (semiconductor device: semiconductor chip) can be reduced.
(b) The circuit arrangement using a combination of first resistors, second resistors, first capacitors and second capacitors can increase the circuit design freedom in the semiconductor chip. The necessity of arranging the first resistor, second resistor, first capacitor and second capacitor limits the arrangement of the elements on the chip.
In addition, the attenuation of the high-frequency signal in the first resistors can be decreased by connecting the bypass capacitors, or second capacitors in parallel therewith, so that the amplifier efficiency can be raised. For example, the same efficiency as in the conventional example 1 can be attained as shown in FIG. 7.
According to the above means (2), too, the efficiency is almost the same as in the case when the second capacitors are not provided, under the condition at which the amplifier gain is saturated, or at the point where the input power is as large as, for example, 25 dBm as shown in FIG. 7. This is because the increase of gain due to the bypassing of the high-frequency signal by the second capacitors not to produce the loss in the first resistors and the increase of the efficiency associated therewith are cancelled out by the gain saturation effect of the amplifier.
Therefore, when the gain of the amplifier in operation is saturated, use of the arrangement shown in FIG. 1, not the arrangement of the means (2), is desired by considering the difficulty of the chip design due to the complexity of arranging the elements and the increase of the chip area due to the increase of the number of elements. When the gain of the amplifier is not saturated, the means (2) can be used, and in this case the efficiency is high.
According to the means (3), connection of inductors can enable the use of negative feedback, thus leading to the stabilization of circuits.
According to the fourth means (4), (a) since the second resistors develop base voltage drop with the increase of base current due to the temperature rise of the whole multi-finger type HBT, the current in all the multi-finger HBT can be suppressed, and thus the thermal runaway can be suppressed.
(b) In addition, since the first resistors attenuate the high-frequency signal, preventing the gain from excessively increasing so that the transistors can be stabilized not to oscillate.
(c) Moreover, since the first resistors develop different base voltage drops for each finger in accordance with the base current increase difference due to the temperature rise difference among the HBT fingers (unit transistors), the collector current difference can be reduced so that each finger can be uniformly operated without current concentration in one finger.
(d) A high-efficiency and small-sized radio communication apparatus can be produced by connecting the first resistors, second resistors and first capacitors in the circuit arrangement. For example, if a high-frequency power amplifier module is constructed by HBT with about 100 transistors and with the emitter size of about 2 xcexcmxc3x9720 xcexcm, it can produce output of about 4 W. At this time, if the current amplification factor is selected to be about 80, an efficiency of about 70% can be attained under the connection of the first capacitors of about 0.15 pF, first resistors of about 100 ohms, and second resistors of about 1 kilo ohms (see FIGS. 7 and 8). At the time of this high-efficiency operation, the DC current flowing in the first resistors is about 0.2 to 0.5 mA, and the voltage drop across the first resistors is as sufficiently small as 20 to 50 mV, so that the loss in those resistors is small. From FIG. 9, it will be seen that the efficiency change is relatively small up to 2.5 times the first resistors of 100 ohms, or 250xcexa9 (parallel resistance of 100 resistors is 2.5xcexa9). The voltage drop across the first resistors at this time is as sufficiently small as 50 to 25 mV. This means that the radio communication apparatus can attain a high efficiency of about 60%.
(e) Only the first resistors are necessary to connect for each HBT in the element arrangement, and the second resistors and first capacitors may be provided for each block that is formed by a plurality of HBTs arranged so that thermal resistance difference can be reduced. Therefore, the freedom of arranging the elements can be increased. In addition, the decrease of the total number of resistors and capacitor elements results in the decrease of the area of the separation regions between the elements. Therefore, it is possible to design a low-cost chip (semiconductor device) with the area reduced.